In packet switching systems having a large number of customers, it is customary to have the customers interconnected to a main packet switching network via digital subscriber interfaces (also called packet concentrators.) Each digital subscriber interface accepts packets from the attached customers via low-speed transmission lines and then retransmits those packets to the main packet switching network via a high-speed transmission link. Prior art systems have utilized multi-customer formatter circuits to assemble individual asynchronous data signals received from a plurality of customers into bytes that are later formed into packets by a program controlled processor and also to accept bytes of asynchronous data from the program controlled processor and to transmit those bytes as individual data signals to the destination customers. It is known in the prior art to loop-around packets for test purposes within a packet switching system. A problem in such loop-around test arrangements is the difficulty in localizing or isolating a failing structural element of a multi-customer formatter involved in a loop-around test. Another deficiency is that the time required to initialize such a customer interface unit greatly impacts the effectiveness of such a maintenance technique.